Phytosociology

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Phytosociology is the study of the characteristics, classification, relationships, and distribution of plant communities. A phytosociological system is a system for classifying these communities. It is often suggested that it is not a science in its formal sense.

The aim of phytosociology is to achieve a sufficient empirical model of vegetation using plant taxa combinations that characterize univocally vegetation units. Vegetation units as understood by phytosociologists may express largely abstract vegetation concepts (e.g. the set of all hard-leaved evergreen forests of western Mediterranean area) or actual readily recognizable vegetation types (e.g. cork-oak oceanic forests on Pleistocene dunes with dense canopy in SW-Iberian Peninsula). Such conceptual units are called "syntaxa" (singular "syntaxon") and can be set in a hierarchy system called "synsystem" or syntaxonomical system. The act of creation, amelioration or adjusting the synsystem is called "syntaxonomy". Therefore, the syntaxonomical system is putatively a sufficient empirical representation of vegetation of a given territory. An International Code of Phytosociological Nomenclature, issuing the rules for naming ‘‘syntaxa’’ exists and its use has increased among vegetation scientists.

The basic unit of syntaxonomy is the "association". The association is a conceptual model of a concrete phytocenosis (the plant component of a biocoenosis). The association is defined by its characteristic combination of plant taxa, habitat features, physiognomy, biogeographical area, role in ecological succession, historical (e.g. history of use by humans) and paleo-biogeographical relationships. Associations with floristic and territorial affinities can be grouped in larger ecological conceptual units (i.e. syntaxa) called "alliances". Similar alliances may be grouped in "orders" and orders in vegetation "classes". The setting of syntaxa in such a hierarchy makes up the syntaxonomical system, or the reference model of the given vegetation and territory.

In spite of early attempts (e.g. Charles Flauhault in the late XIX century), this science started in Europe, with the Swiss botanist and ecologist Josias Braun-Blanquet (1884 - 1980).

Nowadays, phytosociologists try to include higher levels of complexity in the perception of vegetation, namely by describing whole succesional units (vegetation series) or, in general, vegetation complexes. These lie in the scope of Landscape Phytosociology. Other developments include the use of multivariate statistics for the definition of syntaxa and their environmental interpretation.

On the one hand, some authors consider phytosociology to be in the scope of contemporary vegetation science, a successful approach because of its highly descriptive and predictive powers, and its usefulness in nature management issues. On the other hand, there are numerous critics who have focused on several methodological limitations: the absence of statistical approaches, the complexity and non-stability of the nomenclatural system, the mistakes in the predictive models, and certain basic assumptions.

Even if in continental Europe, a complete synsystem describing vegetation types has been developed and it is a basis for habitat-type classification (e.g. NATURA 2000 typology and habitat network), there are numerous scientific experts who do not have a positive opinion about the suitability for phytosociology to be the main geobotanical approach for managing our vegetation systems. An important point of disagreement is the floristic-phytosociological assumption that the forest patches of the Mediterranean species of pines mainly derived from afforestations, non-stables and incidentals.

Phytosociological data contain information collected in relevés (or plots) listing each species cover-abundance values and the measured environmental variables. This data is conveniently databanked in a program like TURBOVEG (Hennekens and Schaminée 2001) allowing for editing, storage and export to other applications. Data is usually classified and sorted using TWINSPAN (Hill 1979) in host programs like JUICE to create realistic species-relevé associations. Further patterns are investigated using clustering and resemblance methods, and ordination techniques like CANOCO (ter Braak and Šmilauer 2002) .

References

Hill MO (1979) TWINSPAN: A FORTRAN Programme for arranging multivariate data in an ordered two-way table by classification of the individuals and attributes. Ecology and Systematics, Cornell University, Ithaca, NY

Hennekens SM, Schaminée JHJ (2001) TURBOVEG, a comprehensive data base management system for vegetation data. Journal of Vegetation Science 12: 589-591

ter Braak CJF, Šmilauer P (2002) CANOCO Reference manual and CanoDraw for Windows User’s guide: Software for Canonical Community Ordination (version 4.5). Microcomputer Power, Ithaca, NY